Not all people born in the same country, speaking the same native language, speak English with the exact same accent. Each person has their own unique speech pattern, pronunciation, intonation, grammar, and usage. Although there may be many generalities and sound substitutions that they have in common, we treat each client’s evaluation as individual to them.
It also important to take into account other influencing factors when evaluating a non-native speaker’s English: where did they learn, and who taught them. If for instance, the English teacher was from England, the client may sound more British rather than American. Another factor may be if they speak a third or fourth language, their pronunciation of English may be influenced in several directions.
Occasionally, a person may have an underlying speech disorder, such as a lisp, which may exist in their native language as well as when they speak English. The strength of the accent also varies from person to person, ranging from mild to severe. So, while there are universal similarities in learning new sounds in speaking a new language, individual differences are the differences that make a difference.